1. Field of Invention
This invention relates to systems and methods for generating binary irrational halftone dots based on a spatially modulated stimulus.
2. Description of Related Art
When creating image regions using halftoning, binary clustered halftone dots are desirable. In particular, binary clustered halftone dots often produce the least amount of noise and the best highlights. Conventional halftoning adds a two-dimensional, spatially periodic, dot screen or line screen structure to the images to be halftoned. Typically, the same screen, or at least a number of essentially identical screens, are used to halftone each of the color image separation layers of a polychromatic, i.e., color, image. However, the halftone screens are oriented at different angles for printing the respective halftone color image separation layers.
Digital halftoning has evolved as a method of rendering the illusion of continuous tone, or xe2x80x9ccontonexe2x80x9d, images using devices that are capable of producing only binary picture elements. However, digital halftoning can suffer from misregistration between the various color image separation layers used in color image, for example, cyan, magenta, yellow and black (CMYK) color image separation layers. This misregistration can be caused by misalignment among the various halftone screens and also by misalignment between the halftone screens and an image forming apparatus grid structure, i.e., an output grid structure, used to generate electronic image data from an image, on an image forming member. This misregistration can also include errors in rotation of the screen angle. This misregistration can cause moir xc3xa9 patterns.
Moir xc3xa9 patterns can also be generated based on the screen angles used for each of the color separation layers, even without any misregistration. Regardless of how the moir xc3xa9 patterns are formed, moir xc3xa9 patterns are detrimental to the accurate rendering of the color image. Moir xc3xa9 patterns arise due to xe2x80x9cbeatingxe2x80x9d, i.e., periodically mismatching patterns of interference that degrade the resulting rendered images. When the various color separation layers are combined during rendering of a multicolor image, where each color separation layer uses a different halftone screen or the same screen at a different angle, a moir xc3xa9 pattern can result. The resulting moir xc3xa9 pattern can cause a color shift or variation in tone.
Substantial effort and expense have been invested in minimizing the moir xc3xa9 patterns caused by halftoning techniques for producing binary renderings of contone images. Misregistration, improper screen angle, and improper screen frequency can increase the halftone screens"" susceptibility to moir xc3xa9 patterns. Additionally, because the moir xc3xa9 patterns can be caused by halftone screens beating with the output grid structure, the moir xc3xa9 pattern may be caused by a difference between the halftone screen, pitch frequencies and the re-sampling rate frequency within the image forming apparatus. Even minor variations in the dot position caused by systematic errors, such as quantization round off errors, can produce moir xc3xa9 patterns resulting from beat frequencies between the periodic screens.
In general, increasing the angle differences between the halftone screens reduces the prominence of moir xc3xa9 patterns because the interference between the image separation layers is more frequent but the amplitude of the interference is lessened. In addition to errors in frequency or in angle, the grid structure of the stimulus applied by the image output apparatus used to create the color separation layers can also contain imperfections. If the respective grid structures for all of the color separation layers do not exactly align, the halftones can be misregistered, becoming another source of moir xc3xa9 patterns.
Thus, the perceived quality of the resulting color image is strongly dependent on the precision with which the color image separations are spatially registered with each other, as well as the precision with which the halftone screens are oriented in relationship to each other and/or to the output grid used by the image forming apparatus. Conventional halftoning methods, such as those disclosed in U.S. Pat. No. 5,410,414 to Curry, incorporated herein by reference in its entirety, and U.S. Pat. No. 4,537,470 to Schoppmeyer, warp, i.e., adjust or move, the image data produced by an image data generator to improve registration. Such image data generators include gray scale image generators and binary image generators. However, merely warping the image data to improve registration results in offsets with the image data which have no corresponding adjustment or warp in the halftone screens used to render the color image separation layers.
Therefore, minimizing the moir xc3xa9 patterns conventionally includes also warping one or more of the halftone screens in a halftone screen system to correspond to the warping of the image data. This is disclosed in greater detail in U.S. Pat. No. 5,732,162 to Curry, incorporated herein by reference in its entirety. The 162 patent provides a detailed discussion of warping both image data and halftone screens.
High addressability or hyperacuity refers to the ability to locate an edge, occurring between one portion of an image and another portion of an image, at a resolution that is greater than the resolution of the stimulus used to form the image. Such edges often occur between halftone dots and the non-image background regions of each of the color separation layers.
One common stimulus used by various image forming apparatus to form images is a light beam scanned by a raster output scanner (ROS). A raster output scanner scans one or more such light beams across a photoreceptor drum or belt. In general, the raster output scanner scans each of the light beams across the photoreceptor drum or belt in a fast scan direction while the photoreceptor drum or belt simultaneously moves relative to the scanned light beam in a slow scan direction. As the one or more light beams are scanned across the photoreceptor drum or belt in the fast scan direction, the one or more light beams are individually modulated between off and on at a high rate. In particular, in various known high addressability systems, each light beam is modulated at a rate that is four times the period it takes the raster output scanner to move the one or more light beams a distance along the fast scan direction that is equal to the diameter of the light beams. This is known as 4xc3x97 high addressability. As shown in FIGS. 1 and 2, 4xc3x97 high addressability allows the location at which the one or more light beams are turned on to be spatially controlled to one-quarter of the diameter of the light beam along the fast scan direction.
However, as also shown in FIGS. 1 and 2, the center-to-center spacing of two adjacent light beams or of two adjacent scans of a single light beam are offset by the diameter of the one or more light beams. Therefore, as shown in FIG. 3, when the edges of an image structure, such as a halftone dot, extend across the laser beam in directions that are not substantially aligned across the fast scan direction, the light beam cannot merely be turned on when the current scan of the light beam intersects with the image structure, such as a halftone dot, and left on until the light beam no longer intersects the image structure. Doing so would result in significantly more toner being applied to the resulting developed image at that area. This would itself result in that portion of the image having an image density that significantly departs from the desired image density represented by the image structure, such as the halftone dot. Conventionally, as shown in FIG. 16, to avoid this change in image density, the edge of the image structure, such as the halftone dot, that extends along the fast scan direction, and therefore, across the slow scan direction, is xe2x80x9cditheredxe2x80x9d, i.e., modulated, at a very high rates, so that the actual amount of image density of the developed image more closely corresponds to the image density of the overall image structure, such as the halftone dot.
It should be appreciated that, in the preceding discussion, and throughout this disclosure, the discussed exemplary embodiments use a flying spot raster output scanner. In such raster output scanners, xe2x80x9chorizontalxe2x80x9d refers to the fast scan direction, while xe2x80x9cverticalxe2x80x9d refers to the slow scan direction. While the following discussion will generally use the terms horizontal and vertical to refer to the fast and slow scan directions, it should be appreciated that there are other types of exposure systems and imagers, such as LED light bar printers or ink jet printers, that switch the directions so that it may be more convenient in such systems to refer to the fast scan direction as the xe2x80x9cverticalxe2x80x9d direction and the slow scan direction as the xe2x80x9chorizontalxe2x80x9d direction. For ease of understanding, the following discussing will use the terms xe2x80x9chorizontalxe2x80x9d and xe2x80x9cverticalxe2x80x9d relative to the fast and slow scan directions. However, those of ordinary skill in the art will readily be able to determine those systems where the horizontal, rather than the vertical, edges will be aligned with the slow scan direction.
Electronic registration refers to adjusting the spatial positions on the image substrate that the image structures, such as the halftone dots, will be placed by the image forming apparatus to compensate for any physical offsets in the image forming apparatus that would otherwise result in misregistration between the color separation layers. That is, rather than physically, i.e., mechanically, ensuring that the color separation layers are precisely aligned, the various offsets between the various color separation layers are measured. The electronic data is then electronically adjusted to change the spatial locations of the resulting image structures, so that the resulting image structures of each corresponding color separation layer are properly aligned. Being able to move or warp an image structure such as halftone dots without causing moir xc3xa9 patterns or noise that detract from the image quality will increase the utility of electronic registration.
Dithering the edges of an image structure, such as a halftone dot, that extend horizontally along the fast scan direction can accurately capture the correct image density to be represented by the image structure, such as the halftone dot. However, such high-frequency structures along the edges of the image structure, such as the halftone dot, often, if not invariably, result in artifacts, such as moir xc3xa9 patterns or noise, when the halftone dots are moved to accomplish electronic registration. In particular, dithering the edges of the image structures, such as the halftone dots, to ensure the proper tone is reproduced generally renders it impossible to avoid such moir xc3xa9 patterns and/or noise in the resulting image when electronic registration is used.
In general, in high resolution image forming apparatus, the halftone dots are formed using a dot-shaped function varies the shape of the halftone dot based on the intensity level to be reproduced. Once the dot-shaped for a particular intensity level is determined using the dot-shaped function, a sample window is used to traverse the resulting dot-shape, as shown in FIGS. 1-3, to determined the modulation of the one or more light beams necessary to reproduce that dot-shape on the photoreceptor drum or belt. It should be appreciated that the dot-shape function can vary. However, one common dot-shape function is a cosine function. For cosine dots, a small eccentricity may be included to spread the midtone dot gain.
The need to create dots at specified angles or the need to warp dots in response to electronic registration requirements results in screen angles that are irrational. An irrational angle occurs when the tangent of the angle cannot be described as a quotient of two small integers. Dithering the edges of such irrational halftone dots, as outlined above, tends to result in significant moir xc3xa9 patterns appearing in the resulting image.
This invention provides systems and methods that permit halftone dots to be warped with reduced moir xc3xa9 patterns.
This invention separately provides systems and methods for warping halftone dots printed at irrational screen angles.
This invention separately provides systems and methods that avoid using dithering when generating image structures having edges that are not substantially perpendicular to a high addressability direction of the grid structure of the stimulus of the image forming apparatus.
This invention separately provides systems and methods that generally gather together, i.e., cluster, portions of an edge of an image element that represent the portions of an image object around an edge of the image object that lies along a high addressability direction and across a non-high addressability direction into generally a single block that generally corresponds to the correct image density.
In various exemplary embodiments of the systems and methods according to this invention, the result of a high resolution integration of an image structure, such as a halftone dot-shape function, is clustered. For fast scan edges of an image, i.e., those vertical edges of the image that extend across a high addressability direction, this is simple. In particular, those fast scan edges tend to be easily determined, as those fast scan edges cross over the edges, i.e., the xe2x80x9chorizontal window edgesxe2x80x9d, of a high addressability sample window that extend along the high addressability, i.e., fast scan, direction. In contrast, when the edge of the image structure, such as the halftone dot-shape, extends across an edge of the high addressability sample window that does not extend along a high addressability direction, i.e., a xe2x80x9cverticalxe2x80x9d window edge, the sample window is incrementally altered, and in particular, is usually extended, in the horizontal direction in one or both directions along the horizontal axis until the edge of the image structure, such as the halftone dot-shape, no longer crosses either vertical window edge of the altered sample window.
The image density of the portion of the halftone dot-shape contained within the altered sample window is then determined. Depending on the image values of the four corner high addressability regions within the altered window, the determined amount of image density is xe2x80x9cfilledxe2x80x9d from the left or right edge of the expanded window. Otherwise, depending on the state of the four corner image regions, a xe2x80x9ccenter of gravityxe2x80x9d of the region of the image structure within the altered window is identified. The center of a block representing the image density of the area of the image structure enclosed within the altered window is then aligned with the determined center of gravity.
Once the block of image density of the portion of the image structure contained within the altered window is aligned with the right edge, the left edge or the xe2x80x9ccenterxe2x80x9d of the altered window, an amount of the determined block that extends into the original sample window is determined. This amount extending into the original sample window determines the amount of image density to be generated in the final output image based on that sample location of the sample window. That amount is recorded in the sample window as a vertical image edge and stored. Any quantization error between the amount of density appearing in the original sample window and the producible amount of image density recorded for the current sample window is determined and stored for future use.
In general, when either edge of the image structure traverses the vertical edges of a sample window, it is generally impossible to determine what value to put inside the sample window without looking at the outside, surrounding context. That is, the xe2x80x9chorizontalxe2x80x9d image edge could be a portion of a descender, a portion of an ascender, a slowly ascending left attached mark, a slowly descending right attached mark, or the like. However, the cluster result must always contribute the right amount of addressability units to the total area of the image structure, and geometrically position that xe2x80x9cright amountxe2x80x9d in reference to the center of gravity of the image structure, regardless of the set of samples that happen to define the image structure. The systems and methods of this invention tends to reduce this problem, such that the cluster result does, in fact, contribute generally the desired amount of addressability units to the total area of the image structure, and positions the clustered result appropriately, regardless of the set of samples that happen to define the dot.
These and other features and advantages of this invention are described in, or are apparent from, the following description of the systems and methods according to this invention.